The technical part of the optician's profession involves mounting a pair of corrective ophthalmic lenses on a spectacle frame selected by a wearer.
This mounting is divided up into three main operations:                acquisition of the contours of the surrounds of the selected spectacle frame,        centering of each lens, which consists in appropriately positioning and orienting the acquired contours relative to the lenses so that, once assembled, each lens is centered on the pupil of the corresponding eye of the wearer, then        machining of each lens, which consists in cutting it along this contour.        
In the context of the present invention, interest is more particularly focused on the spectacle frames with surrounds, that is to say on circular and half-rim spectacle frames.
The concrete objective of the optician is then to cut the ophthalmic lens so that it can be mechanically and aesthetically adapted to the shape of the corresponding surround of the selected frame, while ensuring that this lens best exercises the optical function for which it was designed.
The machining operation comprises in particular, in the case of circular frames, a beveling step used to form on the edge of the lens a fitting rib, commonly called a bevel, capable of being fitted into a groove, commonly called a bezel, which runs along the internal face of the corresponding surround of the frame.
The machining operation however comprises, in the case of half-rim frames, an edge-grooving step with which to form a fitting groove on the edge of the lens. During mounting, this fitting groove is fitted onto a rib that runs along the internal face of the corresponding half-rim (or “arch”) of the frame. The ophthalmic lens is then secured against this arch with the help of a thread that is fitted into the fitting groove and whose ends are connected to the ends of the arch.
The acquisition and machining operations must be carried out with care so that the lens can be perfectly fitted into its surround, without effort and “at the first attempt”, that is to say, without requiring remachining (in the case of circular or half-rim frames) or requiring any bending of the thread (in the case of half-rim frames).
To acquire the form of the surround, a contour reading appliance is generally used which comprises a feeler which records the form of the groove of the surround, or of the groove of a template representative of the form of the surround. Recording errors inherent to the operation of the reading appliance are, however, observed on completion of this feeling operation. Machining errors that are also inherent to the operation of the trimming appliance are also observed on completion of the machining operation.
Despite the care paid to these operations in order to reduce these errors, it is observed that some ophthalmic lenses remain difficult to mount in their surrounds. It is then necessary, to dispel any risk of the lens slipping out from its surround, to remachine the lens and/or modify the length of the nylon thread, which is tedious to do.